The use of transistor devices as signal amplifiers in wireless communication applications is well known. With the considerable recent growth in the demand for wireless services, such as personal communication services, the operating frequency of wireless networks has increased dramatically and is now well into the gigahertz (GHz) frequencies. At such high frequencies, Gallium Arsenide field effect transistors (GaAs FETs) have been preferred for power amplification applications, such as, e.g., use in mobile communication devices to provide power amplification for RF signals. In particular, GaAs FETs have a relatively high saturation power efficiency at frequencies of a few giga-hertz, e.g., at 2 GHZ.
When a GaAs FET is operated as a common source amplifier, such as in a metal-semiconductor field effect transistor (MESFET), the transistor gate is supplied with both the RF input signal to be amplified, as well as a DC bias voltage. Since the gate of a GaAs FET is a Shottky barrier, the relatively strong RF input signal power will rectify the Shottky barrier and generate high positive gate current, which can destroy the transistor device. As a result, the DC bias supply circuitry is conventionally designed to prevent a high gate current.
By way of illustration, FIG. 1 illustrates a conventional power amplifier circuit 10 for amplifying a RF input signal, designated as “RFIN,” with the amplified output signal designated as “RFOUT.” The amplifier 10 includes a GaAs FET 15 operated as a common-source amplifier, with the input signal RFIN applied to the gate terminal, the output signal RFOUT received off the drain terminal, and the source terminal providing a relative ground for the common element current path. The amplifier 10 further comprises a gate bias circuit 20 for coupling a DC source 35 to the gate terminal of the GaAs FET 15. A DC blocking capacitor 25 is used in a conventional fashion to prevent the DC voltage from source 35 from passing upstream along the RFIN signal path.
Within the gate bias circuit 20, the gate bias voltage from the DC source 35 is coupled to the gate of the GaAs FET 15 via a series connected, current limiting resistor 30. In particular, when a high gate current is generated by the RFIN signal, the current will create a voltage drop between the gate and the DC source 35 across the resistor 30 and, thus, lower the gate current. The resistor 30 acts like a negative feedback to control the gate current and, thus, protect the transistor device 15. By way of further illustration, FIG. 2 is a graph of an exemplary gate current IG versus power of the RF input signal RFIN if the signal were connected directly to the bias voltage source 35 without the resistor 30. When the power of the RF input signal RFIN is relatively low, the inherent body Shottky diode of the gate of the GaAs FET 15 is reversed biased and the gate current IG is very small. As the power of the RF input signal RFIN is increased, the gate current IG increases in the negative direction from the drain to gate. This negative gate current IG is caused by drain-to-gate breakdown of the GaAs FET 15. As the power of the RF input signal RFIN is further increased, the gate Shottky diode is rectified, i.e., forward biased, causing the gate current IG to rapidly increase in the positive direction from the gate to the source. Heat generated by this positive gate current IG, if allowed to increase unchecked, can destroy the GaAs FET device 15. Thus, the resistor 30 limits high positive gate current by producing a voltage drop between the voltage bias source 35 and the gate of the GaAs FET 15 when positive gate current flows through the resistor 30.
Returning to the amplifier 10 in FIG. 1, a relatively large value capacitor 40 is connected to ground between the DC voltage source 35 and the resistor 30 to create a ground path for the gate current. A shunt inductance 45 is coupled between the resistor 30 and the transistor gate to prevent the RF input signal RFIN from flowing through the capacitor 40. In the illustrated embodiment, the shunt inductance 45 comprises a quarter-wavelength (¼ λ) stub, where λ is the wavelength of the fundamental carrier frequency f0 of the RF input signal, e.g., 2 GHz, in parallel with a relatively small bypass capacitor 50 shorted to ground. The ¼ λ stub appears as a short circuit to the RF input signal RFIN, while providing a low (essentially purely resistive) impedance path for the DC bias voltage source 35. The bypass capacitor 50 provides a short to ground for the RFIN signal and an open circuit for the voltage bias source 35, and is invisible to the gate current of the GaAs FET 15. Therefore, the ¼ λ stub 45 passes the voltage bias source 35 to the gate terminal of the GaAs FET 15, while blocking the RF input signal RFIN from entering the gate bias circuit 20. Alternately, a RF choke could be used instead of the ¼ λ stub for providing the shunt inductance 45.
Notably, the voltage drop across the resistor 30 varies the gate bias voltage when there is positive gate current. This variation in the gate bias voltage varies the bias condition of the amplifier 10, which impacts the amplifier's performance, e.g., gain, output power, impedance matching, etc. It would be desirable to correct for this non-linear distortion by using known predistortion techniques. However, because the RF input signal RFIN is typically amplitude modulated, a time-varying amplitude envelope is impressed on the RF input signal. When this time-varying amplitude is large enough to forward bias the Shottky diode of the gate of the GaAs FET 15, positive gate current is produced. The variation in the gate bias voltage will depend not only on the instantaneous gate current, but on the history of the gate current leading up to the instantaneous gate current, as well. This phenomenon, commonly known as a “memory effect,” is caused by the voltage bias capacitor 40 forming a RC circuit with resistor 30, which limits the response time of the gate voltage circuit 20 to changes in the gate current. When the frequency of the gate current exceeds 1/(2πRC), the gate bias circuit 20 can not change the gate bias voltage fast enough to follow instantaneous changes in the gate current. As such, the ability of predistortion to correct the distortion of the RF output signal caused by the variation in the gate bias voltage in conventional RF amplifier circuits is limited.